The weather stripping on the roof of my car needs to be replaced. Would anyone know what the approximate cost of this is?
Depends on the car?
Yup. It’s an impossible to answer question without knowing the year, make, and model, as well as if it’s a convertible and exactly what weather stripping you’re talking about.
Parts of the weather stripping are loose on the driver’s and passenger’s sides of the roof, as well as along the windshield. Are there mechanics or auto body shops that replace only parts of the weather stripping on any one side of the roof (as opposed to the entire piece of weather stripping along that side), and is this just as effective in keeping out moisture as replacing the entire weather stripping along that side of the roof?
It still depends on the car and in this case even the shop. Your best bet is to just call a couple shops and see what they can do.
You can’t cut out and replace a section if it wasn’t designed that way in the first place. Weatherstripping has continuous sections up to drip point where a seam will not leak into the vehicle. It wouldn’t save anything even if they could do that, the money is in labor not the stripping itself. They come pre-formed in sections to fit the application…
Make/model/year/body style please (again)…
Guess that’s too big of a question to answer.
’’…on the roof of my car…’'
WHAT CAR ?
They didn’t offer Mind Reading 101 in my school, and even then you had to be in the same room…
Not off somewhere down the ‘‘singing wire’’ of off in cyberspace.
Somewhere between 0 and 1 million dollars.
As you become more specific, so will we. 
Well at any rate, door weather stripping is usually glued on now so it can be re-glued or replaced but the whole thing needs to be replaced, not pieced in. I don’t know what the cost is. Probably $50 or so. Glass shop, body shop, dealer can do it. The windshield weather stripping is something else though. The windshield is glued to the body or weatherstripping around the windshield and then trim piece over that. The trim can be replaced but if it is leaking (correct me if I’m wrong) the windshield needs to be reset or there is rust. Thats a $100 plus job at a glass shop or body shop.